According to the Skin Cancer Foundation, although some approach total protection, no sunscreen is capable of blocking 100 percent of ultraviolet radiation. A sunscreen with a sun protection factor rating, or SPF, of 50 filters out 98 percent of all incoming UV rays.
There are two kinds of ultraviolet radiation that damage skin. UVB radiation is responsible for sunburns. UVA radiation causes premature aging of the skin and is believed to amplify the effects of UVB rays, which increases the risk of skin cancer. Although skin may not be reddened hours after application of sunscreen, UVA radiation can still penetrate the skin and cause damage. Around 1 ounce of sunscreen is necessary for satisfactory sun protection. Sunscreen should be applied 30 minutes before exposure to the sun and every two hours thereafter.